Polyimides are being widely developed because of their good heat resistance and mechanical properties. In particular, wholly aromatic polyimides can achieve especially high heat resistance and excellent mechanical properties due to their stiff structures. Recent trends call for wholly aromatic polyimides particularly having excellent toughness that can endure use in severer conditions.
Aromatic polyimides are generally insoluble and infusible. Therefore, their precursors, i.e., polyamic acid solution compositions, are often used for molding and processing. The polyamic acid can be imidized in various ways, including: (a) thermal imidization; (b) chemical imidization using a dehydrator; and (c) a combination of thermal and chemical imidization. Among these, methods (b) and (c) employing chemical imidization can achieve imidization at relatively low temperatures, but the solution is prone to gelate, thus posing difficulty in producing polyimide-resin-formed products with satisfactory quality. Meanwhile, thermal imidization employed in method (a) involves solvent removal and is thus less prone to cause gelation. It, however, requires heating at elevated temperatures for a prolonged period of time—e.g., raising the temperature stepwise up to the maximum heating temperature while imparting high physical properties during the heating step—in order to produce polyimide-resin-formed products, such as polyimide films, with excellent properties.
Patent Document 1 discloses an aromatic polyimide using 3,3′,4,4′-biphenyltetracarboxylic acid as the tetracarboxylic acid component and 4,4′-oxydianiline and 1,3-bis(4-aminophenoxy)benzene as the diamine components constituting the polyamic acid. Patent Document 2 discloses a fusible-and-moldable crystalline polyimide resin. These Patent Documents, however, fail to disclose mechanical properties thereof.
Patent Document 3 discloses a method of improving the properties of polyimide-formed products obtained after viscosity adjustment and thermal imidization of a polyamic acid solution, the method including: adding an aromatic tetracarboxylic acid or an anhydride thereof to a polyamic acid solution prepared using an excessive amount of an aromatic diamine with respect to an aromatic tetracarboxylic dianhydride, such that the acid component and the diamine component become equimolar. However, the properties, such as the mechanical properties, of the polyimide-formed products obtained according to this method are not necessarily sufficient, and there still is room for improvement to suitably produce polyimide-resin-formed products, such as polyimide films, having properties equal to or superior to those of straight-chain polyimides through heating at relatively low temperatures and/or in a short time.
Patent Document 4 discloses a varnish containing a polyamic acid having an amino group at its molecular end and, as a cross-linking component, a polyfunctional carboxylic acid compound represented by the following chemical formula capable of forming three or four imide rings through reaction with the amino group.

(In the formula, n represents 3 or 4; Z represents a trivalent or tetravalent aromatic group; and R1 and R2 each independently represent a monovalent group selected from hydrogen, an alkyl group, or a phenyl group.)
The varnish of Patent Document 4 may be effective in improving solvent resistance—one of the weak points in so-called thermoplastic polyimides having low glass transition temperatures. However, because of the large number of cross-linking points in the varnish, the resultant cross-linked polyimide tends to become hard and/or brittle, causing problems during use due to deterioration in flexibility, extensibility, and toughness compared to common polyimides. Particularly, with so-called wholly aromatic polyimides having glass transition temperatures of 250° C. or higher and exhibiting excellent heat resistance and mechanical properties due to their stiff structures, there is difficulty in the heating step to suitably control the cross-linking reaction while, at the same time, increasing the molecular weight of the straight-chain polyimide segment to impart high physical properties, thus posing difficulty in suitably producing polyimide-resin-formed products, such as polyimide films, having excellent properties through heating at relatively low temperatures and/or in a short time.
Further, the polyfunctional carboxylic acid compound of Patent Document 4 represented by the above chemical formula has a peculiar structure containing four or more aromatic rings. This compound may be effective in improving solvent resistance—one of the weak points in so-called thermoplastic polyimides having glass transition temperatures of 250° C. or below. However, the segment originating from the polyfunctional carboxylic acid compound will occupy a large volume fraction within the polyimide, thus creating a significant impact on the polyimide properties, such as disturbing the polyimide's crystalline properties, and posing difficulty in making the properties intrinsic to polyimides become evident, especially in highly-heat-resistant polyimide-formed products having glass transition temperatures of 250° C. or higher. Therefore, it is difficult to suitably produce polyimide-resin-formed products, such as polyimide films, having properties equal to or superior to those of straight-chain polyimides through heating at relatively low temperatures and/or in a short time, especially in cases of highly-heat-resistant polyimide-resin-formed products.
Further, the polyfunctional carboxylic acid compound is neither commercially available nor easy to synthesize, thus difficult to obtain and extremely costly.
Non-Patent Document 1 discloses a polyimide using mellitic trianhydride. The Document, however, merely discloses a polyimide consisting of mellitic trianhydride and a diamine, and describes nothing about combining it with a straight-chain polyamic acid.
Patent Document 1: JP-A-61-143433
Patent Document 2: JP-A-63-172735
Patent Document 3: JP-A-60-63226
Patent Document 4: JP-A-2003-41189
Non-Patent Document 1: Shim J. H. et al., Materials Science Monographs (1984), 21, pp. 61-68